Method for producing by vapour-phase epitaxy a gallium nitride film with low defect density

ABSTRACT

The invention concerns a method for preparing gallium nitride films by vapour-phase epitaxy with low defect densities. The invention concerns a method for producing a gallium nitride (GaN) film from a substrate by vapour-phase epitaxy deposition of gallium nitride. The invention is characterized in that the gallium nitride deposition comprises at least one step of vapour-phase epitaxial lateral overgrowth, in that at least one of said epitaxial lateral overgrowth steps is preceded by etching openings either in a dielectric mask previously deposited, or directly into the substrate, and in that it consists in introducing a dissymmetry in the environment of dislocations during one of the epitaxial lateral overgrowth steps so as to produce a maximum number of curves in the dislocations, the curved dislocations not emerging at the surface of the resulting gallium nitride layer. The invention also concerns the optoelectronic and electronic components produced from said gallium nitride films.

This invention relates to the preparation of films made of galliumnitride (GaN) with low defect densities by vapour phase epitaxy.

It also relates to optoelectronic and electronic components made fromthese gallium nitride films.

At the end of 1995, the Nichia Company made a laser diode from III-Vnitrides. This result showed that it is possible to obtain a laseremission from a heteroepitaxial structure in which the dislocationdensity was as high as 10⁸ to 10¹⁰ cm⁻². At the end of 1997, Nichiademonstrated that laser emission for 10000 hours could be obtainedprovided that the structure of the laser diode is made on a good qualityGaN layer. This requires GaN layers produced using the ELO (EpitaxialLateral Overgrowth) technology.

Although it has been asserted for a long time that dislocations in GaNdo not behave as non-radiative recombination centres, it has now beenshown that some dislocations with a screw component actually introducenon-radiative centres and that the component performances are very muchbetter on a better crystallographic quality structure. Thus, the life oflaser diodes based on III-V nitride depends critically on thedislocation density in GaN layers on which structures are made.

All efforts being made at the moment are aimed at obtainingheteroepitaxied GaN with the best possible crystalline quality. This iswhy the ELO (Epitaxial Lateral Overgrowth) technique has been broadlydeveloped for GaN with a large number of variants.

Since solid GaN substrates are not available with a satisfactory surfaceand in sufficient quantity, III-V based nitride components are made byheteroepitaxy on substrates such as sapphire, SiC, Si or other. Thesapphire typically used as a substrate does not have a cleavage plane,which implies that in a laser diode structure based on GaN epitaxied onsapphire, it is difficult to make reflecting facets.

Furthermore., the use of a substrate such as sapphire with a mismatch inthe network parameter and the coefficient of thermal expansion generatesa very high dislocation density in heteroepitaxial layers ofGaN/sapphire.

Regardless of the technology, the density of extended defects(dislocations, stacking defects, inversion domains, nanotubes) does notdrop below 5×10⁸ cm⁻². Dislocations propagate in the growth directionand emerge on the surface where they can be identified by Atomic ForceMicroscopy (AFM) or CathodoLuminescence (CL). These dislocations areharmful in several respects. Firstly, with a high density (more than5×10⁸ cm⁻²), defects degrade electronic mobility and electronicproperties (photoluminescence intensity, life of carriers). Furthermorethe emergence of surface dislocations results in a surface depression(Heying et al., J. Appl. Phys. 85, 6470, 1999). In a laser diodestructure based on GaInN multi-quantum wells (MQWs), the dislocationsdisturb the order of MQWs, and cause non-homogenous light emission.Finally, metals used for pure resistive contacts can also diffusethrough these dislocations and nanotubes.

Different epitaxial lateral overgrowth techniques have been developedfor implementation of ELO: 1) by HVPE (Hydride Vapour Phase Epitaxy), 2)by OMVPE (OrganoMetallic Vapour Phase Epitaxy), 3) by pseudo-sublimationor more precisely CSVT for Close Space Vapour Transport and 4)miscellaneous variants without mask for example using etched substrates.All can be used to obtain GaN layers with dislocation densities of lessthan 10⁷ cm⁻² compared with 10⁸ to 10¹⁰ using the standard technology.However, as we will see later, and as is inherent to the technologyused, zones remain in which the dislocation density remains high, aboveopenings and coalescence joints in a technology with an epitaxy step, atcoalescence joints and in the middle of openings in a two-steptechnology in which a first step is carried out to deposit GaN byepitaxy in openings after masking and etching a dielectric layer(particularly by photolithography) to form these said openings followedby a second Epitaxial Lateral Overgrowth (ELO) step in which lateralgrowth of the initially deposited GaN patterns continues until theircoalescence.

One known variant of the growth technology is based on OrganometallicVapour Phase Epitaxy (OMVPE) using a process that has now been welldefined (on sapphire): surface treatment on sapphire, low temperaturenucleation of a GaN or AIN layer, annealing of this nucleation layeruntil the final growth temperature and growth of GaN at high temperature(1000-1100° C.) Several technologies were developed to optimise thisheteroepitaxy and to limit the dislocation density in GaN to about 5×10⁸cm⁻² (coalescence of islands of GaN, Haffouz et al., Appl. Phys. Lett.,73, 1278 (1998), X. H Wu. et al, Jpn J. Appl. Phys., 35, L1648 (1996)).

The low temperature nucleation layer is no longer necessary on SiC, andthe first step is to make an AIN layer at high temperature before theGaN is deposited. However, the dislocation density remains approximatelyof the order of 5×10⁸ cm⁻².

Thus, as presented above, epitaxial lateral overgrowth (ELO) and itsmany variants forms one of the most relevant methods of reducing thedislocation density by several orders of magnitude, in other words toless than about 10⁷ cm⁻².

The following describes how defect lines propagate in GaN firstly whenthe ELO process with one-step epitaxy is used, and secondly when thetwo-step process is used, to better understand the invention.

One-Step Epitaxy Process

The first step is to epitaxy a first layer of GaN on a substrate, and adielectric mask is then deposited on this layer. The next step is toperform photolithography of openings in this dielectric mask withclearly defined dimensions and crystallographic orientations. Epitaxy iscontinued on GaN layers thus prepared firstly in the openings; thisresumed epitaxy causes lateral growth of GaN crystals which has theeffect of reducing the dislocation density by several orders ofmagnitude. Through dislocations do not propagate above the mask.However, GaN that is epitaxied from the openings, consistent with theinitial GaN, maintains the same dislocation density as the initialcompound. Furthermore, lateral patterns with a low dislocation densitycoalesce and, since the initial GaN is in a mosaic pattern, the weakdisorientation leads to a region with a high dislocation density in thecoalescence plane or the coalescence joint. Consequently, it isimpossible to use the entire surface to manufacture optoelectroniccomponents if a one-step ELO is used.

FIG. 1 diagrammatically shows this one-step epitaxy process. A GaN layeris epitaxied (GaN base layer 2) on a substrate 1. A mask 3 (SiO₂,SiN_(x), Al₂O₃, W, etc.) is then deposited (by CVD, PACVD, cathodicsputtering, sublimation, in situ CVD or any other deposition method).Openings are formed on this mask by photolithography, along clearlydefined crystallographic directions and with appropriate dimensions, forexample 3 μm openings separated by 7 μm along the [1-100]_(GaN)direction. When GaN growth is resumed, the deposition takes placefirstly in the openings 5, then laterally above the mask 4. Above theopenings, GaN in epitaxial contact with the substrate, maintains thesame defect density as the base layer 2. The black lines in FIG. 1represent dislocation lines. The GaN laterally grows above the mask(overgrowth of GaN 4). Through dislocations do not propagate in thiszone, as is established in the state of art. However, a coalescencejoint 6 is formed when the two lateral overgrowth fronts join in themiddle of the mask. Therefore the manufacturing technology for a laserdiode on an ELO substrate as described above requires a complextechnology since the diodes structure has to be made on overgrowth zones4, between the coalescence joint and the zone in epitaxial contact withthe substrate, which requires an alignment precision of the order of oneμm.

Two-Step Epitaxy Process

This variant is an improvement to the one-step epitaxy process. It isshown diagrammatically in FIGS. 2, 3 and 4.

FIGS. 2 and 3 are analysed as follows:

After epitaxying a GaN base layer reference 2 on a sapphire substratereference 1, an in situ deposit of SiN is made (masks 3), and openings 5are then etched by photolithography along clearly definedcrystallographic directions. The final step is to resume growth whichfirstly leads to selective epitaxial overgrowth 6.

During the first resumed epitaxy, growth conditions are adjusted toobtain a higher growth rate along direction <0001> than the lateralgrowth rate, such that overgrowth in the form of strips with atriangular section with facets {11-22} is obtained. The advantage ofthis procedure is to induce curvature of emerging dislocations at 90° asillustrated in FIG. 4.

This dislocation curvature is explained by energy considerations. Theforce acting on a dislocation line is the sum of two terms:

one makes this line curved so that it remains normal to the surface,

the other tends to align the dislocation line with the Burgers vector(to minimise the dislocation formation energy).

In the second step, the experimental conditions are modified to obtain alateral growth rate greater than the growth rate along the <0001>direction to obtain total coalescence. FIG. 3 shows an intermediate stepin which facet (0001) 7 reappears.

This two-step process is described particularly in patent applicationWO99/20816. The modification of experimental conditions to obtain alateral growth rate higher than the growth rate along the <0001>direction may consist of adding magnesium, antimony (Sb) or bismuth (Bi)to cause anisotropic GaN growth (L. Zhang et al, Appl. Phys. Lett., 79,3059 (2001).

This technology provides a means of obtaining GaN with dislocationdensities less than or equal to 10⁷ cm⁻² (over the entire surfacebetween coalescence zones) (Vennéguès et al, J. Appl. Phys. 87, 4175(2000)).

There are regions with almost no observable defects between thecoalescence zones as can be seen in the images of the surface incathodoluminiscence presented in FIG. 5, in which part (a) of the figureis an image of a GaN layer obtained by a two-step epitaxy process andpart (b) is an image of a GaN layer obtained by a one-step epitaxyprocess.

These zones with a low defect density are sufficiently wide to makeoptoelectronic components such as laser diodes. A careful examination ofthese images shows that there is a significantly higher density of blackdots (emergence of dislocations) at approximately the centre of stripsdefined by coalescence zones, than in the rest of the strip with a lowdefect density. These dislocations have their origin in the GaN baselayer, located in the middle of the openings, which after the growthstep emerge near the vertex of triangular overgrowths, and which thusescape the dislocation curvature process. During ELO growth, afterselective epitaxy, experimental conditions are such that the facets{11-22} begin to form, and through dislocations at the edge of the maskcurve first. FIG. 4 gives a good understanding of this phenomenon.Dislocations in the middle of the mask can escape from this process andemerge on the surface (dislocation A). Furthermore, after curvature, thedislocations propagate parallel to the base plane. The two lateralovergrowth fronts meet and create a coalescence joint. The dislocationsthat follow the lateral growth front can either terminate in thecoalescence zone (in which there may be a void) or they may curvetowards the substrate, or they may curve at 90° and emerge on thesurface. This coalescence joint in which the through dislocation densityis high also limits the useable surface of the ELO substrate.

Therefore, it is clear that this two-step epitaxy process cannoteliminate, all dislocations and particularly dislocations originating inthe middle of the masks and coalescence joints.

To complete the description and to give a good understanding of thecontext of the invention described below, we will now describe thepropagation of dislocation lines. The following description isparticularly applicable to dislocations that originate in the middle ofthe mask openings.

FIGS. 6 and 7 illustrate the case in which dislocations might emerge onthe surface. We will refer to this figure throughout the remainder ofthe description when mentioning the different types of symmetry that canbe encountered (a), (b) or (b′).

FIG. 7 illustrates the case in which the pattern 4 has a trapezoidal ortriangular section and the mask 3 have a common axis of symmetry.Emergent dislocations are located at the vertex of the triangularsection 4 and coalescence joints 6 form vertical planes. The environmentof a dislocation for which the line coincides with the common axis ofsymmetry is firstly an (a) configuration then (b) then (a) again duringgrowth: it is never curved. Similarly, the environment of a dislocationof the coalescence joint during growth is a type (b′) configuration then(a); there is no curvature of its line.

Other variants of the ELO vapour phase technology use textured orperiodically etched substrates (Ashby et al. Appl. Phys. Lett. 77, 3233(2000) instead of a dielectric mask. In these technologies, etchings aremade directly in the substrate, which avoids a growth step anddeposition of a mask.

This technique cannot eliminate all dislocations and particularlydislocations originating from the middle of openings and coalescencejoints.

Therefore, there is an urgent need to find technical solutions to thisproblem of dislocations emerging on the surface of GaN films that reducethe useable surfaces of GaN films for manufacturing optoelectroniccomponents, regardless of the process adopted for making the film usingone, two or even several epitaxy steps or etching openings directly inthe substrate.

The purpose of the invention is to propose a process for making a GaNfilm that provides a GaN film with low defect densities.

Note that in the context of this invention, the GaN may or may not bedoped. Doping substances include particularly magnesium, zinc,beryllium, calcium, carbon, silicon, oxygen, tin and germanium. It isalso possible to introduce an isoelectronic impurity such as In, Sc, Sb,Bi among the elements in column III or V in the Mendeleev periodictable.

Thus, the purpose of the invention is a process for making a film ofgallium nitride (GaN) starting from a substrate, by depositing GaN byvapour phase epitaxy, characterised in that the GaN deposit comprises atleast one vapour phase epitaxial lateral overgrowth (ELO) step, and inthat at least one of these ELO steps is preceded by etching of openings:

-   -   either in a previously deposited dielectric mask,    -   or directly in the substrate,

and in that an asymmetry is introduced into the dislocations environmentduring one of the ELO steps so as to cause the largest possible numberof dislocation curvatures, since curved dislocations do not emerge atthe surface of the GaN layer thus obtained.

The asymmetry of the dislocations environment may be inducedparticularly:

(1) by varying growth parameters either by applying an electric fieldperpendicular to the growth axis, or by illuminating using a lampproducing UV radiation at about 170 to 400 nm, to cause preferentialgrowth of a single family of facets {11-22}, or

(2) by making openings with unequal widths or with unequal geometry,either in the dielectric mask or directly in the substrate to applygeometric shapes to the GaN patterns to facilitate the curvature ofdislocations, or in other words making use of specific properties ofdifferent geometric shapes that can be taken on by GaN patterns duringresumed growth.

This asymmetry provides a means of taking action of most throughdislocations originating from mask openings. Consequently, they nolonger emerge on the surface.

In particular, the purpose of the invention is a process like thatdescribed above, characterised in that asymmetry is introduced by makingopenings either in the dielectric mask or directly in the substrate,that are adjacent, unequal and asymmetric forming a basic pattern of aperiodic network, the basic pattern comprising at least 2 openings,these openings possibly being of different types and particularly lines,hexagons, triangles or a combination of such openings. Preferably, theperiodic network defined above extends along a [10-10] direction.

The ELO technology according to this invention is known by the acronymALFAGEO (Asymmetric Lateral Facet Grown—Epitaxial Overgrowth).

The epitaxial lateral overgrowth step(s) is (are) advantageously made byvapour phase epitaxy from chlorides and hydrides (HVPE), byOrganoMetallic pyrolysis in Vapour Phase Epitaxy (OMVPE), or by CSVT(Close Space Vapour Transport).

It is also possible to perform these epitaxial lateral overgrowth (ELO)steps along one or of the M(1-100), A(11-20), R(1-102), S(10-11) andN(11-23) planes of the substrate, so as to eliminate the piezoelectricfield that exists when epitaxy is done along the C(0001) plane.

The substrates may be about a hundred micrometers thick, usually of theorder of 200 μm, and may chosen from among sapphire, ZnO, 6H—SiC,4H—SiC, 3C—SiC, GaN, AIN, LiAiO₂, LiGaO₂, MgAlO₄, Si, HfB₂ or GaAs. Thesubstrates may be treated before any deposition of GaN by nitridation.

The invention also relates to any GaN film that could be obtained by theprocess according to the invention. The GaN film thus obtained may bebetween 1 and 100 μm thick. According to one particular embodiment ofthe invention, the GaN film obtained may be between 5 and 15 μm thick.

An optoelectronic component is also proposed, and particularly a laserdiode, a photodetector or a transistor, characterised in that it isprovided with a GaN film that could be obtained by the process accordingto the invention.

Thus, according to a first variant of the invention openings are made ina dielectric mask, and according to a second variant of the inventionthe openings are made directly in the substrate.

When the openings are made in the dielectric mask, namely according tothe first variant, the process advantageously comprises a two-stepepitaxial lateral overgrowth (ELO) using the technique described above.

One of the purposes of the invention is thus to propose a process formaking a GaN film that provides a GaN film in which the density ofdislocations originating from the middle of the openings and coalescencejoints are strongly reduced in the case in which the two-step epitaxyprocess is adopted for production of the said GaN film.

The case in which facets {11-22} grow at a different rate so as to causecurvature of the dislocation lines or cases in which openings in themask cause different surface facets are illustrated in FIGS. 8 and 9. InFIG. 8, t₀ denotes the first instant at which the dislocation may becurved, and at t₁ (previous t₀) the dislocation is in a symmetricalenvironment configuration such as (a) for (c) and (d) or such as (b) for(e) and (f). A t₊₁, the dislocation propagates in the base plane (0001).Configuration (c) is the configuration used in the two-step ELOdescribed above. In configuration (d), t₁ and t′⁻¹ show two possiblegeometries leading to the same shape at t₀ .

In the remainder of the description, reference is made to this Figurewhen mentioning typical asymmetry cases (c), (d), (e) and (f).

In case (1) above, the applied asymmetry is according to case (e) and incase (2) the applied asymmetry is according to cases (d) and (f).

The asymmetry in application of configuration (d) may also beneficiallyeliminate coalescence joints, such that the entire ELO surface can beused for manufacture of optoelectronic components.

The dielectric masks that can be used to make this variant of theprocess according to the invention may be composed of silicon nitride(SiN), SiO₂ or W. The dielectric is deposited according to techniqueswell known to those skilled in the art.

The first deposition of GaN may be made by any vapour phase depositionmethod, namely HVPE (Hydride Vapour Phase Epitaxy), pyrolysis inOrganometallics Vapour Phase Epitaxy (OMVPE) or Close Space VapourTransport (CSVT). OMVPE will be used in preference. The gas vector ispreferably a mix of N₂ and H₂. Other vapour phase epitaxy technologiescan also be used for this first layer such as MBE, cathodic sputteringor laser ablation.

A layer of GaN obtained according to the process described below canadvantageously be used for masking followed by resumed epitaxy, from abase layer of GaN.

The substrate is covered by a thickness of silicon nitride approximatelyequal to one atomic plane. After the dielectric mask has been formed, alayer of GaN is deposited, called the continuous buffer layer. Thethickness of this layer may be between 20 and 30 nm. The temperatureduring this operation may be between 300 and 900° C. The next step ishigh temperature annealing at between 950 and 1120° C. The buffer layerchanges from a continuous layer to a discontinuous layer formed of GaNpatterns, or in other words GaN patterns in the form of islands. Thezones in which the dielectric has been exposed then act like a mask andthe GaN patterns act like GaN zones located in openings made ex situ inthe mask. After deposition and annealing of the nucleation layer, a thinlayer of GaN, typically 2 to 5 μm thick, is deposited by Organometaillicpyrolysis in Vapour Phase Epitaxy. The gallium source isTrimethylgallium (TMGa) and the nitrogen source is ammonia. Such amethod is described in many documents. This technique is describedparticularly in patent application WO99/20816, in example 5 that isincorporated herein by reference.

Using this base layer of GaN has the advantage of limiting thedislocation density at the beginning of the process according to theinvention.

The following describes different possible embodiments of the firstvariant of the invention, that are intended to illustrate the inventionand do not limit its scope.

All embodiments described below relate to two-step ELO processes likethose described above.

Thus, the invention more particularly relates to a process for making aGaN film, characterised in that the GaN deposition that follows theformation of openings is broken down into two-step epitaxy, the firstbeing done under growth conditions such that the growth rate along the<0001> direction is greater than the lateral growth rate and the secondbeing done under modified experimental conditions such that the lateralgrowth rate is greater than the growth rate along the <0001> directionso as to obtain full coalescence of the patterns.

The modification of growth conditions such that the lateral growth ratebecomes greater than the growth rate along the <0001> direction consistsof adding magnesium, antimony and bismuth.

According to a first embodiment, adjacent unequal asymmetrical openingsare made to form the basic pattern of a periodic network preferablyalong a [10-10] direction. Examples of such asymmetric opening patternsare shown in FIG. 10. The asymmetric basic pattern is not limited tolinear openings, it would be possible to imagine many other patternssuch as hexagonal openings parallel to the [10-10] directions ortriangular openings. The basis of the invention is to induce propagationof dislocations by asymmetry of the openings that leads to a greaterreduction of their density than in the ELO.

After making these asymmetric openings, treatment of the epitaxied,masked and etched substrate, for example as shown in FIG. 10 underdeposition conditions, is resumed by epitaxy of gallium nitride so as toinduce deposition of gallium nitride patterns on facing zones andanisotropic and lateral growth of the said patterns, lateral growthbeing continued until coalescence of the said patterns.

For example, FIG. 11 diagrammatically shows the variation of themorphology during ELO of GaN when the widths of the openings areunequal.

During the first step, growth conditions are chosen such that the (0001)plane is a fast plane. This first step terminates when the (0001) planehas disappeared, all GaN patterns obtained by growth from unequalopenings then reach a triangular section; the section of the GaN patterncorresponds to the thick black line delimiting the two separate greyareas in FIG. 11.

During this first step (dark grey area delimited by the black line inFIG. 11), through dislocations are curved at 90° when they meet thelateral facets {11-22} during growth (such that N is at point 4 inconfiguration (c)). Dislocations located at the exact mid-point of smalland large openings are not curved (denoted M1 and M2) and continue topropagate vertically beyond this first step. Similarly, if patternsalready coalesce at this stage as is the case in FIG. 11, dislocationssuch as N′and N″ converge towards the coalescence joint (denoted C1),and propagate vertically beyond this first step. The result is a voidformed in the middle of the masks.

In the second step, in which growth conditions are modified, the facets(0001) reappear. This second ELO step consists of resumed epitaxy bychanging the growth conditions to change the growth anisotropy so thatit becomes conducive to planarisation of GaN patterns. As described inWO 99/20816, this can be achieved either by adding magnesium in thevapour phase, or by increasing the temperature. In this second step, GaNpatterns develop with an expansion of the facet (0001) (which reappearsat the vertex of each pattern) while the surface of the lateral facetsreduces. Due to the asymmetry of the pattern, the dislocations M2 of thesmall openings and C1, C2 of the coalescence joints emerge in thelateral facets {11-22} in the type (d) configuration at points 2, 1 and3 respectively, in which they are subjected to a curvature at 90°. Inthis embodiment, the small number of type M1 dislocations are notcurved. On the other hand, the large number of C2 type dislocations arecurved at 3 in the base plane and can interact and cancel each otherout. FIG. 12 illustrates this behaviour of the dislocations, and thebehaviour of type N, N′and N″ dislocations of large openings, smallopenings and type C1 openings respectively that are curved at 1 can beidentified.

According to a second embodiment, unequal openings are used differentlyfrom the way in which they are used in the first embodiment.

FIG. 13 illustrates an example embodiment of this second embodiment.

As in the first embodiment, growth takes place in two steps that aredifferent in their growth conditions.

But for this second embodiment, the first step terminates when the GaNpatterns originating from unequal openings in the mask have completelycoalesced to form a single pattern with a triangular section. Theintermediate geometries that can be observed during the first step areindicated in a black dashed line in FIG. 13.

In the second step, growth conditions are chosen to achieveplanarisation by making the base plane C (0001) reappear as shown in agrey dashed line in FIG. 11.

Type C1 and M2 dislocations are curved for reasons mentioned in thedescription of the first embodiment.

This second embodiment is different from the first in the behaviour oftype M1 and C2 dislocations. Type M1 dislocations are curved at point 1because, at this point, M1 is in a (c) configuration. On the other hand,type C2 dislocations are not curved.

In a third embodiment, three unequal openings are used.

The previous two embodiments allow one dislocation type: M1 in the firstembodiment and C2 in the second embodiment. These first two embodimentscan be combined into a third, so that type M1 and type C2 can both becurved. Once again there are two steps with different growth conditions.This third embodiment is illustrated in FIG. 14.

During the first step, the GaN patterns originating from unequalopenings O1 and O2 coalesce to form a pattern with a single triangularsection and M1 dislocations are curved; this is the same as the secondembodiment. At the same time, the pattern originating from opening O3,located sufficiently far from O2, develops to reach a triangularsection. The end of the first step coincides with obtaining a profileshown with a black line in FIG. 14; this is the same as the firstembodiment profile. Grain joints (C3 in FIG. 14) are curved at 6.

In a fourth embodiment, asymmetry is introduced during growth.

As mentioned in the introduction, asymmetry may also be created byilluminating the side of the substrate during growth with UV radiationso as to increase the growth rate of a single family of facets {1-212}.An electric field can also be applied perpendicular to the direction ofthe openings. Asymmetry is introduced into the growth starting fromsymmetrical etched patterns (or unequal patterns to combine effects),and after coalescence at the end of the first step (or at the beginningof the first step), by increasing the growth rate of one of twoequivalent facets {11-22} (for example by illuminating the side of thestructure with a UV laser, or by applying an electric fieldperpendicular to the directions of the openings).

Through dislocations M located in the middle of the mask are not curvedin the first phase of the ELO, on the other hand they are curved at 1(FIG. 8(e) when asymmetry is introduced into the facet growth rate{11-22}. The result of the asymmetry is a coalescence joint that is nolonger perpendicular to the surface of the substrate, such that thedislocations, after being curved at 90°, join together in thecoalescence joint. Some of the dislocations stop in this joint, in whichthere is often a void, and one part propagates downwards and anotherpart propagates vertically, denoted C. These parts meet a facet {11-22}at 2, and are curved at 90°.

When the openings are etched directly in the substrate, namely accordingto the second variant, the step for formation of the GaN base layer maybe done under the same conditions as described above, in other wordswhen the first variant of the process is implemented.

Similarly, this second variant may advantageously Comprise two lateralovergrowth steps (ELO) that may be done under the same conditions asdescribed above, in other words when the first variant of the process isimplemented.

The characteristics, purposes and advantages of the invention will alsobecome clear after reading the following example of a particularembodiment of the invention and the attached figures in which:

FIG. 1 represents a one-step epitaxy;

FIG. 2 represents a first step in a two-step epitaxial lateralovergrowth;,

FIG. 3 represents a second step in a two-step epitaxial lateralovergrowth;

FIG. 4 shows the variation of the structure before total coalescence.The dislocations propagate parallel to the base plane. The dashed linesrepresent the different possible shapes of the ELO patterns at the endof the first step;

FIG. 5 shows a set of two images of the surface in cathodoiuminiscence.Each black dot corresponds to emergence of a through dislocation. Part(a) of the image represents a GaN surface produced according to thetwo-step epitaxy process and part (b) of the image represents a GaNsurface produced according to the one-step process. The diameter ofthe * mark is 20 μm;

FIG. 6 represents 3 example configurations in which the dislocationpropagates in an environment that remains symmetric during growth (solidbold lines t₀ and dashed lines t₁ show two positions at successive timesof planes C in (a) and (11-22) in (b) and (b′);

FIG. 7 represents the case of symmetrical growth in which the overgrowthof GaN and the opening 5 have a common axis of symmetry;

FIG. 8 represents cases of asymmetrical growth;

FIG. 9(a) shows a case of asymmetrical growth obtained when the leftfacet grows faster that the right facet; 4 and {3 and 5} havediscontinuous axes (or planes) of symmetry. All dislocations originatingfrom the opening of the mask are in configuration (c) or (e) at a givenmoment. Curvature will occur;

FIG. 9(b) represents a case of asymmetry obtained by choosing an unequalshape for openings 5 a and 5 b; the overgrowths 4 a and 4 b coalesce toform a ribbon 4 c for which the plane of symmetry A4 does not coincideby construction with any of the other planes of symmetry (A1, A2, A3).All dislocations originating from openings in mask 5 a and 5 b or thatpropagate vertically above the mask 3 b, are in configuration (c) at agiven moment. There will be curvature;

FIG. 10, (a) represents a mask with openings along a [1-100] directionwith unequal width openings, and (b) and (c) represent a mask withopenings along the two type [1-100] directions;

FIG. 11 represents a diagrammatic view of a two-step ELO processstarting from openings in the mask with unequal widths. The first stepis shown as a thick black line and the second planarisation step isshown as a dashed line;

FIG. 12 represents the structure of through dislocations in a GaN layermade by a two-step ELO process, starting from asymmetric openings. Thesection of two patterns coalesced during the first step is shown as awhite line. Dislocations curved at 90° are identified, and no type Mdislocation is observed in the smallest pattern. The {11-22} facet thatis developed during the second step is shown in dashed grey lines. Thetype C dislocations that originate from coalescence joint are curved at90° when they meet this facet (point 2).

FIG. 13 represents the variation of GaN patterns when the process isimplemented according to the second embodiment described above;

FIG. 14 represents the variation of GaN patterns when the process isimplemented according to the third embodiment described above.

EXAMPLE

The first part of the example has been taken from example 1 in WO99/20816.

An appropriate vertical reactor is used operating at atmosphericpressure for Organometallic pyrolysis in Vapour Phase Epitaxy. A thinlayer of gallium nitride (2 μm thick) is deposited on a 200 to 500 μmthick sapphire substrate (0001), by Organometallic pyrolysis in VapourPhase Epitaxy at 1080° C. The gallium source is trimethylgallium (TMGa)and the nitrogen source is ammonia. Many documents describe such amethod.

The experimental conditions are as follows:

The gas carrier is a mix of equal quantities of H₂ and N₂ (4 sl/mn).Ammonia is added through a separate pipe (2 sl/mn).

After growth of the first epitaxial layer of gallium nitride, a thinlayer of silicon nitride is deposited in the growth chamber. Asymmetricopenings are formed in the dielectric by photolithography, with 1 μm and2 μm openings (mask in FIG. 10(a)). The linear openings areadvantageously oriented along a [10-10] direction of GaN although theprocess described in this example can eventually be carried out forother orientations of openings particularly along the [11-20] directionof GaN.

Epitaxy is resumed on zones exposed using GaN not intentionally dopedunder operational conditions of the first resumed epitaxy in thetwo-step process such that the growth rate along the [0001] direction ofGaN patterns is sufficiently greater than the growth rate along thedirection normal to the inclined sides of the said patterns. Under theseconditions, growth anisotropy causes disappearance of the (0001) facet.The first step in use of the process terminates when the (0001) facet ofthe GaN pattern disappears. At the end of the first step, the patternsare in the shape of strips with a triangular section (with lateralfacets with orientation {11-22} or {1-101} depending on whether theinitial strips were oriented along [10-10] or [11-20]), with unequalsize (FIG. 12).

The second step consists of resuming epitaxy by GaN by modifying thegrowth anisotropy (by increasing the temperature to 1120° C. or byadding magnesium in the form of a volatile organometallic form (MeCp2Mg)in the vapour phase). The TMGa flow is 100 μmole/minute. The (0001)facet reappears at the vertex of each GaN pattern obtained in the firstphase. These GaN patterns then develop with expansion of the (0001)facets and, on the contrary, a reduction in the flanks. Due to theasymmetry of the triangular patterns, two adjacent flanks (originatingfrom different sized patterns) coalesce before total coalescence of allpatterns. In this variant of the ELO, the coalescence zone (or thecoalescence joint) of two patterns is no longer a plane parallel to theopenings but is a plane inclined at an angle determined by the ratiobetween the growth rates along the c axis and laterally. The second stepterminates when all flanks have completely disappeared, the uppersurface of the deposit formed by the coalesced patterns of GaN thenbeing plane.

Use of the process according to the invention as described resultsfirstly in obtaining a plane GaN layer, that can therefore be used as asubstrate for the subsequent deposition of the component structure,particularly the laser diode structure, by resumed epitaxy, but alsoleads to a very advantageous improvement in the crystalline quality ofthe said substrate. The lines of dislocations originating from thesubjacent GaN layer propagate through openings formed in the maskvertically in the patterns created in the first step. But it is foundthat the dislocation lines are curved at 90° in a second step.

FIG. 12 shows a high resolution electronic microscopy image of the layerthus obtained and the dislocations are curved at 90° above each openingwhen they meet facets {11-22} during the growth. All that can escape atthe beginning of this growth phase are dislocations that originate inthe centre of the mask. Defect lines then propagate along directionsparallel to the surface of the masked GaN layer.

1. Process for making a film of gallium nitride (GaN) starting from asubstrate, by depositing GaN by vapour phase epitaxy, characterised inthat the GaN deposit comprises at least one vapour phase epitaxiallateral overgrowth (ELO) step, and in that at least one of these ELOsteps is preceded by etching of openings: either in a previouslydeposited dielectric mask, or directly in the substrate, and in that anasymmetry is introduced into the dislocations environment during one ofthe ELO steps so as to cause the largest possible number of dislocationcurvatures, since curved dislocations do not emerge at the surface ofthe GaN layer thus obtained.
 2. Process for making a film of galliumnitride GaN according to claim 1, characterised in that asymmetry isinduced: (1) by varying growth parameters either by applying an electricfield perpendicular to the growth axis, or applying a magnetic field, orby illuminating using a lamp producing UV radiation at about 170 to 400nm, to cause preferential growth of a single family of facets {11-22},or (2) by making openings with unequal widths or with unequal geometry,either in the dielectric mask or directly in the substrate to applygeometric shapes to the GaN patterns to facilitate the curvature ofdislocations.
 3. Process according to claim 1 or 2, characterised inthat asymmetry is introduced by making openings either in the dielectricmask or directly in the substrate, that are adjacent, unequal andasymmetric forming a basic pattern of a periodic network, the basicpattern comprising at least 2 openings.
 4. Process according to claim 3,characterised in that the openings are lines, hexagons, triangles or acombination of these openings.
 5. Process according to claim 3 or 4,characterised in the periodic network extends along the [10-10]direction.
 6. Process according to any one of claims 1 to 5,characterised in that epitaxial lateral overgrowth (ELO) step(s) is(are) made by vapour phase epitaxy from chlorides and hydrides (HVPE),by OrganoMetallic pyrolysis in Vapbur Phase Epitaxy (OMVPE), or by CSVT(Close Space Vapour Transport).
 7. Process according to any one ofclaims 1 to 6, characterised in that the epitaxial lateral overgrowth(ELO) step(s) are done along one of the C(0001), M(1-100), A(11-20),R(1-102), S(10-11) and N(11-23) planes of the substrate.
 8. Processaccording to any one of claims 1 to 7, characterised in that thesubstrate is chosen from among sapphire, ZnO, 6H—SiC, 4H—SiC, 3C—SiC,GaN, AIN, LiAiO₂, LiGaO₂, MgAlO₄, Si, HfB₂ or GaAs.
 9. Process accordingto claim 8, characterised in that the substrate is a sapphire substrate.10. Process according to any one claims 1 to 9, characterised in thatthe gallium nitride is doped during at least one epitaxial lateralgrowth in vapour phase using a doping substance that can be chosen fromamong magnesium, zincs beryllium, calcium, carbon, silicon, oxygen, tinand germanium.
 11. Process according to any one of claims 1 to 10,characterised in that an isoelectric impurity such as In, Sc, Sb, Bi isintroduced in the gallium nitride.
 12. Process according to any one ofclaims 1 to 11, characterised in that the openings are etched in adielectric mask.
 13. Process according to claim 12, characterised inthat before deposition of the dielectric mask, a GaN base layer is madeby vapour phase epitaxy from chlorides and hydrides (HVPE), byOrganoMetallic pyrolysis in Vapour Phase Epitaxy (OMVPE), or by CSVT(Close Space Vapour Transport).
 14. Process according to claim 13,characterised in that the formation of the GaN base layer comprises thefollowing steps: deposition of silicon nitride with a thicknessapproximately equal to one atomic plane, deposition of a GaN bufferlayer, high temperature annealing at between 950 and 1120° C., such thatthe buffer layer changes from a continuous layer to a discontinuouslayer formed of GaN patterns in the form of islands, then, deposition byepitaxy of GaN.
 15. Process for making a film of gallium nitride (GaN)according to any one of claims 12 to 14, characterised in that theprocess comprises two separate vapour phase epitaxial lateral overgrowth(ELO) steps, the GaN deposition during the first step is made in the GaNzones located in the openings, and the GaN deposition during the secondstep leads to lateral overgrowth until coalescence of the GaN patterns.16. Process according to claim 15, characterised in that the GaNdeposition during the :first step is made under growth conditions suchthat the growth rate along the <0001> direction is greater than thelateral growth rate, and the GaN deposition during the second step ismade under modified experimental conditions such that the lateral growthrate is greater than the growth rate along the <0001> direction so as toobtain full coalescence of the patterns.
 17. Process according to claim16, characterised in that the modification of the growth conditions toobtain a lateral growth rate higher than the growth rate along the<0001> direction consists of adding magnesium, antimony or bismuth. 18.Process according to any one of claims 1 to 11, characterised in thatthe openings are directly etched in the substrate.
 19. Process accordingto claim 18, characterised in that this process is implemented accordingto operational conditions described in claims 14 to
 17. 20. Galliumnitride film, characterised in that it may be obtained using a processaccording to any one of claims 1 to
 19. 21. Gallium nitride filmaccording to claim 20, characterised in that it has a thickness ofbetween 1 and 20 μm.
 22. Optoelectronic component, characterised in thatit is made from a GaN film according to either claim 20 or
 21. 23. Laserdiode, photodetector or transistor, characterised in that it is madefrom a GaN film according to either claim 20 or 21.